This invention relates to an apparatus for induction heating of billet-shaped blanks of electrically well conductive and non-magnetic metal, in particular aluminium or copper, comprising a winding adapted to surround the blank completely or partially, to be supplied with electric alternating current from a power supply and to be cooled by means of a cooling system at least during the heating of the blank.
A known and typical arrangement for such induction heating is shown in FIG. 1 in a simplified and schematic manner. There is shown a workpiece or blank 100 and a cut-through winding 101 which is adapted to be supplied with alternating current as shown. Moreover, with dashed lines there is illustrated at least partially how a generated magnetic field passes through the blank 100 for the heating thereof.
When the material in blanks of interest is an electrically well conductive and non-magnetic metal, such as aluminium and copper, the prior art induction heating devices have a low efficiency, namely a maximum of about 50%. In other words about one half of supplied electric power will get lost in the windings. Moreover induction heating of aluminium and copper blanks, including the so-called billets, is characterized by a high capacity per unit volume. In typical installations there is the question of capacities of 500 kW. Accordingly, within this field there is a strong desire of obtaining improvements for the purpose of energy economy and resource savings.
An additional factor of interest in this context is that the blanks concerned of aluminium or copper exist in the form of extrusion billets, which have a relatively elongate shape and are usually solid. Thus, these are per se with respect to their main shape, well suited for induction heating.
An actual example of induction heating may be found in U.S. Pat. No. 5,781,581. This primarily relates to a chamber (xe2x80x9csoaking pitxe2x80x9d) for cooling and re-heating of parts having just been cast. In this case the material apparently is steel. The parts are placed in the chamber, which is adapted to be evacuated. There are mounted radiation screens or the like in order to prevent heat from escaping and getting lost. In the case of a need for re-heating, induction effect is employed, or as an alternative direct heating by providing for a current flow through the blank or workpiece. The frequency range in induction heating is stated to be in the range of 100-1000 Hz. These frequencies indicate that there is here the question of a magnetic material (steel) that is to be treated. In the patent specification and as a subordinate feature there is included a short reference to superconduction as a possible effect of interest.
Another example of prior art having a certain interest in this connection, is U.S. Pat. No. 5,391,863. Superconduction is not mentioned in this patent specification, that otherwise has a content corresponding to some degree to the first paragraph of the present description.
In this connection there is reason to note that superconductors have been known for a long time and also at least from more than 10 years ago based on cooling with liquid nitrogen. The present invention comprises an advantageous utilization of superconductors, as will appear from the following description.
In an arrangement for induction heating of solid, cylindrical billets as illustrated in FIG. 1, the efficiency is determined by the following formula:   η  =      1          1      +                                    ρ            v                                              ρ              b                        ⁢                          μ              b                                          
where xcfx81v and xcfx81b are the resistivities of the material in the winding and the blank or billet, respectively, and xcexcb is the relative permeability of the billet material. Whereas the permeability of iron is of the order of magnitude of 1000, it is approximately equal to 1 for non-magnetic materials such as aluminium and copper. This means that iron is substantially more favourable with respect to the efficiency in such induction heating. When the blank or billet consists of a non-magnetic material with very good electrical conductivity, as for example aluminium or copper, the efficiency will be about 50%, since the resistivities for a traditional copper winding and the blank material respectively, are approximately equal. In other words the value of the root expression in the formula above, will be approximately equal to 1. Thus, one half of supplied electric power will be consumed in the induction winding and one half will be transferred to the blank.
Substantial improvements in the above relationships will be obtained according to the invention in an apparatus for induction heating as referred to at the beginning of this description, in that the winding has turns comprising superconducting material and is surrounded by a thermally insulated chamber, that the cooling system is adapted to keep the winding at a temperature in the range of 30-90xc2x0 K, and that the frequency of the alternating current is adapted to be within the range of common mains frequencies.
In an advantageous embodiment of the apparatus according to the invention, the cooling in the cooling system takes place with liquid nitrogen or helium gas being brought to circulates in cavities or cooling channels adjacent to the winding inside the thermally insulated chamber. Nitrogen has a boiling point of 77xc2x0 K at normal atmospheric pressure and in actual practice it can be appropriate to keep the winding temperature 10-12xc2x0 lower than this boiling point, when liquid nitrogen is used. On the other hand there will often in practice be a somewhat higher temperature in the actual winding than the temperature of the cooling medium. When using nitrogen at 77xc2x0 K the winding temperature therefore may be at 90xc2x0 K. A winding temperature of 60xc2x0 K will be optimal in many instances. Suitable temperatures in this connection will to a significant degree depend upon the materials employed in the winding, in particular the superconducting materials. When helium is used, the temperature range should be between 40xc2x0 and 60xc2x0 K. Below 40xc2x0 K the cooling costs will be significantly increased.
In another possible embodiment there is according to the invention, provided a jacket of well heat conducting, but electrically insulating material being in thermal contact with the winding, and being cooled by means of a cooling unit which is a part of the cooling circuit of the cooling system.
The embodiments just referred to show that windings comprising superconductors require quite different design solutions from what is tranditionally found in electric induction heating. Usual structures with copper conductors involve hollow conductors, so that cooling water can circulate through the hollow conductors in the winding. With the low (cryogene) temperatures being of interest according to the invention, there will be the question of quite different solutions for the cooling. Accordingly, the thermal insulation will also be more significant. Moreover, it is a feature of interest that certain types of superconducting threads have anisotrope properties in so far as the losses depend upon the direction of the magnetic field in the winding.
A substantial advantage with respect to efficiency consists therein that this will increase from about 50% to 90-95%. This of course is very significant and shows that there is here the case of a new solution having a high practical value for the industry.